1. Field of the Invention
This invention relates to devices for metering perforated film in a roll film camera and, more particularly, to such devices that sense the presence of a film perforation and selectively lock and release the film transport mechanism.
2. Description of the Prior Art
Devices for metering successive roll film exposures by arresting the film transport mechanism in response to the sensing of perforations or notches precut in the film at specific intervals are old and well known in the art. Commonly assigned U.S. Pat. No. 3,148,605 which issued Sept. 15, 1964 in the names of Dean M. Peterson et al discloses a camera film transport mechanism including a sensing pawl which detects the film perforation and a metering lever which is operable to disable the transport mechanism. Upon sensing the arrival of the perforation at a predetermined location, the sensing pawl is moved a predetermined distance and in so moving releases the metering lever which operates to disable the transport mechanism. In that patent, the shutter release lever is operative to release a high energy lever which is associated with the shutter mechanism and further acts to withdraw the sensing pawl from the perforation. It has been found that, occasionally, the pressure on the film from the sensing pawl, coupled with vibrations in a camera when a high energy lever is rapidly moved, will cause film movement during the exposure interval.
The risk of film movement can be reduced by leaving the sensing pawl in a film perforation during exposure operation of the camera. Such a mechanism is disclosed in U.S. Pat. No. 3,736,854 issued June 5, 1973 in the name of David E. Beach. That patent discloses a mechanism in which the sensing pawl is not removed from the perforation during operation of the shutter mechanism, but remains in the film perforation until after exposure operation and until transport mechanism has begun to advance film. At that time, the sensing pawl is cammed out of the film perforation by cooperation between a cam surface on the sensing pawl and a ramp fixedly mounted in the camera. A return spring acts on the sensing pawl after removal from the film perforation to return the sensing pawl to its perforation sensing position. However, the force of the return spring must be carefully controlled during manufacture or that spring may, in conjunction with undesired vibrations and other forces in the camera, result in metering problems or film motion during exposure operation.
In a pawl-in mechanism, as disclosed in U.S. Pat. No. 3,736,854, the sensing pawl is biased against the trailing edge of the film perforation by the return spring. A strong return spring may overcome gate friction during exposure operation and cause film movement. Reduction of the spring force on the sensing pawl will minimize the tendency for the film to move during exposure operation. However, this may lead to other metering problems. When the sensing pawl is removed from a film perforation and it is desired to return the pawl to its sensing position, the light spring which is urging the pawl in such a direction may not be of sufficient size to overcome the frictional resistance between the sensing pawl and the film surface. The frictional resistance is directly related to the force with urges the sensing pawl into its sensing position in which it detects the presence of a metering perforation in the film. Even if the sensing pawl is moved to its sensing position, continued film movement may drag the sensing pawl to its metering position prematurely.